Removal of unreacted diisocyanate monomer from polyurethane prepolymers

A process for reducing the amount of residual organic diisocyanate monomer in a polyurethane prepolymer reaction product mixture which comprises heating the polyurethane prepolymer reaction product mixture in the presence of a combination of at least one inert first solvent with a boiling point below the boiling point of the residual organic diisocyanate monomer selected from the group consisting of dimethyl glutarate, dimethyl adipate, diethyl adipate, diisopropyl adipate, ortho-, meta-, or para-dichlorobenzene. 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, .gamma.-butyrolactone, and dimethyl succinate, and at least one inert second solvent with a boiling point above the boiling point of the residual organic diisocyanate monomer selected from the group consisting of dimethylphthalate, diethylphthalate, and an isomer of dibutyladipate, at a temperature which exceeds the vaporization temperature of the residual organic diisocyanate monomer and which is below the decomposition temperature of the polyurethane prepolymer, wherein the combination of the first solvent and the second solvent comprises about 5 to about 85 percent by weight of the total weight of the polyurethane prepolymer reaction product mixture plus the solvents, and wherein the ratio of the first solvent to the second solvent is from about 20:1 to about 1:20.